A known type of multilayer stack for giving the substrates such properties consists of at least one metallic layer, such as a silver layer, which is placed between two coatings of dielectric material of the metal-oxide type. The stack is generally obtained by a succession of deposition steps carried out using a vacuum technique, such as sputtering, optionally assisted by a magnetic field. Two very thin metal layers may also be provided on either side of the silver layer, the subadjacent layer acting as a tie layer for nucleation and the overlayer as a protective layer or “sacrificial” layer so as to prevent degradation of the silver if the oxide layer which is on top of it is deposited by sputtering in the presence of oxygen.
Stacks of this type, having one or two base layers of silver, are thus known from European Patents EP-0 611 213, EP-0 678 484 et EP-0 638 528.
Also known from Patent EP-0 847 965 is a stack consisting of two silver layers, designed so as to be able to undergo a heat treatment of the bending or toughening type without any substantial optical change, thanks to the use of oxygen barrier layers of the silicon nitride type and of layers for stabilizing the silver layers.
Finally, a stack consisting of two silver layers of very different thicknesses is known from Patent EP-0 844 219, making it possible to obtain glazing having a solar factor Lowered to at Least 32% (the solar factor SF is the ratio of the total energy entering a room through the glazing in question to the incident solar energy).
In general, but more particularly in the field of double glazing for buildings, it is advantageous to be able to adjust the light transmission Level of the glazing within a certain range without correspondingly having to completely reconfigure the stack of thin layers each time.
Solutions have already been proposed to meet this objective: in Patent FR-2 751 666 it is proposed to insert, between the glass and the first dielectric layer, an absorbent layer based on iron oxide. In Patent FR-2 708 262 it is proposed to insert an absorbent layer of the titanium nitride type, in contact with and above the silver layer. However, these solutions have a drawback in both cases if the stack of thin layers undergoes a heat treatment of the annealing, bending or toughening type: the absorbent layer will be significantly changed optically and/or make the multilayer stack in its entirety change optically.
This is because if the absorbent layer is in contact with the glass, or with the silver, it will, under the effect of heat, have a tendency to be oxidized, to deteriorate or to cause the adjacent layers to deteriorate in a more or less controllable manner. Thus, if the absorbent layer is in direct contact with the silver layer, it tends to destabilize it by oxidizing. If it is in contact with the glass, the layer will be modified by the diffusion of alkali metal ions coming from the glass.